In a memory management unit, to satisfy systems requiring the transparent mapping of several large areas of memory, it is sometimes necessary to provide memory access control over the remaining address space where the transparent translation registers do not map. This was accomplished in the past by a set of hardwired access control attribute bits stored within the memory management unit. This hardwired storage method has the one major disadvantage that it provides no flexibility, since the attribute bits are stored in a storage location fixed in the hardware and cannot be changed by a system user or programmer.
Some systems incorporated a privilege-mode dependent mode of operation for the MMU to provide access control over the unmapped space. Specifically, in the Identity Translation Mode, access control is regulated by a set of four area descriptors wherein a first descriptor is used to control supervisor space instruction accesses, a second descriptor is used to control supervisor space operand accesses, a third descriptor is used to control user space instruction accesses, and a fourth descriptor is used to control user space operand accesses. This method has a major disadvantage in that it requires the segmenting of the logical address space into supervisor and user spaces since, the supervisor area descriptors cannot protect the supervisor space from unauthorized accesses by user programs. Another major disadvantage this method presents is that it requires a multiple number of register to be configured by the programmer.